Metro certifies documents for the entire $5.6-billion project, but Beverly Hills leaders threaten to sue if the plan to tunnel under the high school proceeds.

Los Angeles County transportation officials set the stage Thursday for a showdown with Beverly Hills leaders over a small portion of the much-anticipated Westside subway extension.

Officials on Thursday certified environmental documents for the entire $5.6-billion project, moving a step closer to construction of nine miles of rail that would mostly run underneath Wilshire Boulevard.

But the Los Angeles County Metropolitan Transportation Authority board only formally approved the first 3.9 miles of the project — as far west as La Cienega Boulevard — because of a request for a hearing from the city of Beverly Hills, where many school officials and city leaders hope to derail efforts to build part of the line underneath Beverly Hills High School.

"It's important to know that the citizens of Beverly Hills have been and remain enthusiastic supporters of the Westside Subway Extension," Beverly Hills Mayor William Brien said. "Our only objection to the project is the route to Century City that would tunnel underneath the historic core of Beverly Hills High School."

Brien said he worries that tunneling underneath the school could potentially be dangerous because of nearby earthquake faults and added that "the tunnel could make building and modernizing of our school facilities extremely costly and thus could preclude future construction over the tunnel."

Metro officials disagree and said their research shows that tunneling underneath the high school is safe and necessary if the subway's route is to include a stop in the heart of Century City.

"There really is differing opinions on the science and it needs to be reconciled," Brien said.

Brian Goldberg, president of the Beverly Hills Unified School District board, said it has spent more than $2 million on the issue and warned that "a lawsuit is absolutely on the table" if, despite the hearing, Metro officials proceed without picking another route.

"All we are asking for is a fair hearing," Goldberg said. A date for the meeting has not yet been set.

Metro board member and county Supervisor Zev Yaroslavsky, who approves tunneling underneath the high school, said that many of those in Beverly Hills advocating a different route are not basing their arguments on fact.

"My commitment has been from Day 1 that Metro should make its decision based on science and facts, not based on hysteria and emotion," Yaroslavsky said earlier this week. "They're just not telling the truth."

The route approved Thursday would leave from the existing Purple Line subway station at Wilshire Boulevard and Western Avenue and head west to the Veterans Administration hospital. If the route is built, officials promise a 25-minute one-way trip between downtown Los Angeles and Westwood.

"We are now closer than we've ever been to getting this project under construction," said Los Angeles Mayor Antonio Villaraigosa, who also heads Metro's board. "This long-awaited subway extension will provide a faster, more reliable transit alternative for hundreds of thousands of Angelenos on a daily basis."

The Metro board also certified the environmental documents for the $1.37-billion downtown regional connector, which will for the first time make it possible to take suburb-to-suburb, one-train rides from Long Beach to Montclair, or from the San Gabriel Valley to Santa Monica.

The connector is a fully underground, 1.9-mile light-rail line that connects the Gold Line to the Blue and Expo lines. Officials said three new stations along the line would provide access to 88,200 commuters, including 17,700 new riders.